The invention relates a tire pressure sensor for motor vehicles. It is known from DE-OS 28 32 447 to screw in a tire pressure sensor at the circumference of a wheel rim, which tire pressure sensor comprises a pressure switch for monitoring the air pressure in the tire and is actuated by the tire pressure. The pressure switch includes, on the one hand, an electrically conducting diaphragm, which closes a reference pressure chamber in the direction of the tire, and, on the other hand, a contact pin which is sealed in a glass bushing so as to be electrically insulated and is contacted by the middle area of the diaphragm when there is sufficient air pressure in the tire in order to close an electric circuit. The reference pressure chamber is filled to a desired reference pressure with air or nitrogen via a duct and is then closed so as to be pressure-sealed. The reference pressure is selected in such a way that the diaphragm contacts the end of the contact pin when there is sufficient air pressure in the tire. On the other hand, if the air pressure in the tire falls below a minimum pressure which is predetermined by means of the reference pressure, the diaphragm rises off the contact pin and opens an oscillating circuit, which comprises a coil and a capacitor, which is arranged in the tire pressure sensor. By means of a high-frequency oscillation of the evaluating circuit, which is arranged so as to be fixed in the vehicle, it can be determined whether or not the pressure switch is closed with a signal receiver arranged in the proximity of the tire when passing the tire pressure sensor with each revolution of the wheel.
In the known constructions of such a tire pressure sensor, it is a disadvantage that a pressure compensation occurs in the reference pressure chamber when there is a leak in the reference pressure chamber, e.g., at the clamping point of the diaphragm, wherein the electric circuit remains closed by the diaphragm so that the information "tire pressure normal" is transmitted by the tire pressure sensor even when the air pressure is too low. A corresponding indication error also occurs if a leak occurs in the reference pressure chamber at the filling duct or at the bushing of the contact pin.